Such devices for introducing sheets of paper are already known for use in printers, typewriters or other apparatuses such as photocopying machines. As retaining member, these devices use very often retaining corners, from under which the upper sheet is extracted when being fed. Though these retaining corners may prove satisfactory for use with sheets which are relatively thin and which are not rigid, it is not possible to use envelopes or sheets which are thicker and more rigid with this type of retaining members.